The present invention relates to a semiconductor laser module which is suitable for the use in particularly high temperature environments, and a Thermo-module (TEC: Thermo Electric Cooler) used in the semiconductor laser module.
Recently, a great number of semiconductor lasers have been used as a light source for signals and a pumping light source for an optical fiber amplifier in optical transmissions. Where the semiconductor laser is used as signal light source and a pumping light source in optical transmissions, it is frequently used as a semiconductor laser module. The semiconductor laser module is a device in which a laser beam from a semiconductor laser is optically coupled to an optical fiber.
FIG. 6 shows one example of structures of such a semiconductor laser module. A semiconductor laser module 40 illustrated in FIG. 6 is such that in a package 11, Thermo-module 42 is fixed on the bottom 11a of a package. A substrate 16, on which a semiconductor laser element 13, a thermistor 14 and a lens 15 are fixed, is fixed on the Thermo-module 42. Also, an optical fiber 17 is fixed in a throughhole 11c secured at a sidewall 11b of the package 11. In FIG. 6, 50 indicates a heat sink.
The semiconductor laser module 40 has a function by which a laser beam emitted from the semiconductor laser element 13 is condensed by using the lens 15 and is made incident into the end face of the optical fiber 17. Subsequently, the laser beam is propagated in the optical fiber 17 and is provided for a specified usage.
In the semiconductor laser module 40, an electric current is fed to drive the semiconductor laser element 13, whereby the temperature of the semiconductor laser element 13 is increased by the generation of heat. The temperature rise will become a cause from which changes in the oscillation wavelength and optical output of the semiconductor laser element 13 results.
Therefore, a thermistor 14 is fixed in the vicinity of the semiconductor laser element 13, which measures the temperature of the semiconductor laser element 13. Using a value measured by the thermistor 14, the electric current fed into a Thermo-module 42 is controlled, whereby the temperature of the semiconductor laser element 13 is kept at a required value by the current control, and the characteristics of the semiconductor laser element 13 are stabilized.
The Thermo-module 42 used in the semiconductor laser module 40 has, generally as shown in FIG. 7A, P type thermoelectric converting elements 18 being a P type semiconductor and N type thermoelectric converting elements 19 being an N type semiconductor 19. The P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are disposed alternatively in a row, and are arranged between two insulation layers 12a and 12b, for example, consisting of ceramic. The P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are electrically connected to each other in series. By application of a direct current voltage to the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19, heat is conveyed to or absorbed on the surfaces of the insulation layers 12a and 12b, whereby an object is heated or cooled.
FIG. 7A shows a cross section of the Thermo-module 42. The Thermo-module 42 is such that P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are placed between two ceramic-made insulation substrates 12a and 12b made of alumina or aluminum nitride. These thermoelectric effect elements 18 and 19 are electrically connected to each other by electrodes 12 formed on the surface of the insulation substrates 12a and 12b. 
FIG. 7B is a perspective view of a Thermo-module 42 illustrated with the insulation substrates 12a and 12b omitted. The Thermo-module 42 is formed so that a number of thermoelectric converting elements 18 and 19 are two dimensionally uniformly disposed on the insulation substrates 12a and 12b. 
FIG. 7C shows an electric connection of the respective thermoelectric effect elements 18 and 19, wherein the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are alternatively connected in series.
The number of thermoelectric elements 18 and 19 to be connected changes in compliance with application. Such that, for example, the number of pairs of the p type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 being from 20 through 40 may be used in a semiconductor laser module.
Such a Thermo-module 42 may be produced as shown below. First, an ingot is produced of material powder mainly consisting of bismuth (Bi) and tellurium (Te) by a single crystallizing method or a hot-pressing method. And, the ingot is cut like chips to produce the P type thermoelectric converting elements 18 and N-type thermoelectric converting elements 19. (For example, this is a publicly known technology disclosed by Japanese Laid-Open Patent Publication Nos. 202343 of 1989 and 106478 of 1989).
Next, as shown in FIG. 8A, a plurality of electrodes 12c are installed on the insulation substrate 12a, and at the same time soldering paste 12e is coated on the respective electrodes 12c. Next, as shown in FIG. 8B, the chip-like P type thermoelectric converting elements 18 are placed one by one on the respective electrodes 12c. Thereafter, as shown in FIG. 8C, the above chip-like N type thermoelectric converting elements 19 are placed one by one on the respective electrodes 12c, whereby the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are disposed alternatively.
And, as in FIG. 8A above, a plurality of electrodes 12c are installed in the insulation substrate 12b, and at the same time, soldering paste 12e is coated on the respective electrodes 12c. And, as shown in FIG. 8D, the insulation substrate 12b having the electrodes 12c provided are arranged on the insulation substrate 12a on which the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are placed. The arrangement is carried out so that the electrodes 12c secured on the insulation substrates 12b are bridged on the electrodes 12c secured on the insulation substrates 12a. That is, adjacent electrodes 12c on the upper insulation substrate 12b are, respectively, put on the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 on the electrodes 12c of the lower insulation substrates 12a. 
In this state, soldering paste 12e is reflown in a soldering reflow furnace (not illustrated). By reflow, the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are bonded between two insulation substrates 12a and 12b, and at the same time, the P type thermoelectric converting elements 18 are electrically connected to the N type thermoelectric converting elements 19 in series via electrodes 12c. And, a Thermo-module 42 shown in FIG. 8E can be produced by the above production process.
The reason why heating and cooling actions can be produced by feeding an electric current to a Thermo-module are described below. That is, as described above, the P type thermoelectric converting elements 18 and N type thermoelectric converting elements 19 are placed between two insulation substrates 12a and 12b, and are electrically connected to each other in series. Therefore, as shown in FIG. 7A, by application of a direct current voltage from outside the Thermo-module 42, an electric current flows from the insulation substrate 12a toward the insulation substrate 12b in the P type thermoelectric converting elements 18, and flows from 12b toward 12a in the N type thermoelectric converting elements 19.
However, holes are majority carriers in the p type thermoelectric converting elements 18, and electrons are majority carriers in the N type thermoelectric converting elements 19. Respectively, transfer of particles carrying the electric current occurs in a direction from the insulation substrate 12a through the insulation substrate 12b. On the other hand, the holes and electrons carrying the electric current also carry heat. Therefore, a heat flow constantly occurs in one direction while the electric current flows in directions opposite to each other in the p type thermoelectric converting elements 18 and N type thermoelectric converting elements 19. Accordingly, cooling is carried out at one side of the Thermo-module 42 and heating is carried out at the other side thereof.
A semiconductor laser module 40 shown in FIG. 6 has such a Thermo-module 42 as described above. A description is given of a thermal environment where the semiconductor laser module 40 is in operation.
In a case where the semiconductor laser module 40 is incorporated in a transmission device such as an optical fiber amplifier, etc., there are many cases where semiconductor laser modules are used at a higher temperature environment than room temperature, due to heating of other semiconductor laser modules and electric circuit elements, etc., which are simultaneously incorporated therein, and specified environments where the transmission device is installed. Therefore, the semiconductor laser module 40 is usefully fixed on a heat sink 50 to be used with an efficient heat dissipation.
FIG. 9 is an exemplary view showing a thermal environment where the semiconductor laser module 40 is fixed at the heat sink 50 and used thereat. Further, in the same drawing, a lens 15 and an optical fiber 17 are omitted for the sake of description.
As shown in FIG. 9, it is assumed that the temperature of an environment where the semiconductor laser module 40 is placed is Ta, and the temperature of the heat sink 50 is Ths. Herein, an electric current is supplied to the semiconductor laser element 13 while keeping the temperature Ts of the thermistor 14 constant. In this case, the amount of heat QLD generated by the semiconductor laser element 13 is transmitted through the substrate 16 and reaches the insulation substrate 12a at the cooling side of the Thermo-module 42, and is exhausted to the insulation substrate 12b at the heating side.
Simultaneously, the amount of heat QTM generated in the Thermo-module 42 itself by the current flowing therein is exhausted to the insulation substrate 12b. Subsequently, the amount of heat (QLD+QTM) is exhausted to the heat sink 50 via the base plate 11a of a package 11.
Where the temperatures of the insulation substrate 12a at the cooling side of the Thermo-module 42 and of the insulation substrate 12b at the heating side thereof are, respectively, TC and Th, the thermal impedance between the semiconductor laser element 13 and the insulation substrate 12a at the cooling side is K1, and the thermal impedance between the insulation substrate 12b at the heating side and the heat sink 50 is K2, the following expressions can be established.
Th=Ths+K2(QLD+QTM)xe2x80x83xe2x80x83(1)
Tc=Tsxe2x88x92K1QLDxe2x80x83xe2x80x83(2)
Therefore, xcex94T=Thxe2x88x92Tc, which is a temperature difference between the insulation substrates 12a and 12b of the Thermo-module 42 can be expressed by expression (3) below:
xcex94T=(Thsxe2x88x92Ts)+(K1+K2)QLD+K2QTMxe2x80x83xe2x80x83(3)
In the expression (3), (Thsxe2x88x92Ts) which is the first term of the right side indicates a temperature difference between inside and outside of the semiconductor laser module 40 where the Thermo-module is used therein.
That is, since there exists a thermal impedance K2 between the insulation substrate 12b at the heating side and the heat sink 50 by the above expression (1), the temperature Th of the heating side substrate 12b of the Thermo-module 42 becomes higher by K2(QLD+QTM) than the temperature Ths of the heat sink.
Further, since there exists a impedance K1 between the semiconductor laser element 13 and the insulation substrate 12a at the cooling side by the expression (2), the temperature Tc the insulation substrate 12a at the cooling side becomes lower by K1 QLD than the thermistor temperature Ts (temperature of the semiconductor laser element).
Thereby, where the Thermo-module is used in a semiconductor laser module 40, a temperature difference (Thsxe2x88x92Ts) between the inside and outside of the semiconductor laser module is decreased by (K1+K2) QLD+K2 QTM in comparison with the temperature difference xcex94T between the insulation substrates of the Thermo-module 42.
Objects and Summary of the Invention
Recently, demand has grown for the abovementioned semiconductor laser modules to operate, at higher optical output and at a higher environmental temperature, in line with an increase in output of the entire system.
As a semiconductor laser element is devised to match a high output in line with an increase in output of the semiconductor laser module, the heat generation amount (QLD) is necessarily increased. In order for such a high output semiconductor laser module to be used at a high temperature environment, it is necessary to further efficiently exhaust heat generating from the semiconductor laser elements than previously.
However, in the abovementioned semiconductor laser modules, there are the following problems.
A description is based on a semiconductor laser module 40 shown in FIG. 9. Amount of heat QLD generated from the semiconductor laser element 13 is exhausted to the heat sink 50 via a substrate 16, a Thermo-module 42, and a package base plate 11a. 
Herein, since the entire thickness of the semiconductor laser module 40 is designed to be thin to a predetermined thickness due to being mounted in a device, the substrate 16 and package base plate 11a are also designed to be thin. Therefore, while heat QLD generated from the semiconductor laser element 13 is being transmitted through the substrate 16 in its thickness direction, and while the heat passes through the package base plate 11a and is exhausted to the heat sink 50, the above heat QLD does not sufficiently spread in the lateral direction (in a direction parallel to the plane of the insulation substrates 12a or 12b of the Thermo-module).
Also, a transfer of heat by the Thermo-module 42 does not principally spread in the lateral direction since the transfer is carried out via holes and electrons inside thermoelectric converting elements.
That is, the heat QLD generated from the semiconductor laser element 13 is concentrated directly below the semiconductor laser elements and in the vicinity thereof, flows and is exhausted to the outsides. Therefore, effective thermal impedance of a channel of the exhausted heat is made greater in comparison with a case where it is assumed that heat is sufficiently spread and uniformly dissipated.
This indicates that the effective values of K1 and K2 are significant in expressions (1) through (3). Where a semiconductor laser module is used in such a situation where the effective thermal impedance of the exhausted heat is significant, a temperature difference generated due to the thermal impedance is made greater in comparison with a case where the heat is uniformly dissipated.
And, the temperature difference is made remarkable in line with an increase in the heat QLD generated from the semiconductor laser element 13 and with an increase in the temperature of the environment where the semiconductor laser module 40 is used (that is, where the QTM is significant).
As a semiconductor laser module 40 is used in such a situation, the temperature difference xcex94T between the insulation substrates 12a and 12b at the heating side and cooling side of the Thermo-module 42 is made very large. And, the load on the Thermo-module 42 is increased, power consumption is also increased, whereby this further increases the environmental temperature, resulting in a vicious cycle.
Therefore, in order to highly increase the output of the semiconductor laser module and to use the same under a further higher ambient temperature, it is necessary to make uniform the heat flow from the semiconductor laser element 13 to the heat sink 50. Accordingly, spreading of heat in the lateral direction is attempted by increasing the thickness of components existing in a channel of heat exhaust, such as a substrate 16, a package base plate 11a, etc. However, since the thickness of the components described above is limited by conditions of mounting a semiconductor laser module in a device, such a measure cannot be employed in reality.
The present invention was developed to solve a problem of heat exhaust existing in the prior art semiconductor laser module described above and to enable the use of the semiconductor laser module at a higher optical output and in a further higher temperature environment. More specifically, the invention is to provide a mounting structure for a semiconductor laser module comprising the semiconductor laser module fixed on amounting substrate. The semiconductor laser module in the mounting structure in one view comprises a package, wherein the package houses a semiconductor laser chip, a chip mounting substrate mounted with the semiconductor laser chip and a Thermo-module for cooling the chip mounting substrate inside thereof; the Thermo-module is placed and fixed on the bottom wall of the package and supports the chip mounting substrate; and a thermal diffusion sheet member is laid between the bottom surface of the package and the surface of the mounting structure, having thermal conductivity anisotropy where thermal conductivity is higher in the surface direction than in the thickness direction.
Additionally, the invention is to provide a mounting structure for a semiconductor laser module in another view, the semiconductor laser module in the mounting structure comprises a package, wherein the package houses a semiconductor laser chip and a chip mounting substrate mounted with the semiconductor laser chip inside thereof; the chip mounting substrate mounted with the semiconductor laser chip is supported by the bottom wall of the package; and a thermal diffusion sheet member is laid between the bottom surface of the package and the surface of the mounting structure, having thermal conductivity anisotropy where thermal conductivity is higher in the surface direction than in the thickness direction.